I have two gentoo machines that no longer shutdown properly and simply hang at unmounting tmp - by that point I've got no console access so can't investigate further. I've searched the forums (including using Google) but can't find anyone else with this issue, however I've now got two machines doing it and I'm concerned that having to reset/unplug the machine to get it to reboot once it hangs is going to cause disk corruption before long.

and that's it, it just sits there, as if it is waiting for a process to release an open file on /tmp - however, without console access at that point I can't check what process it is...

tmpfs is compiled into the kernel.

I've just updated to kernel 3.6.11 to allow udev 197 to work - if the problem persists I might try adding lsof/fuser of /tmp to the unmount of local filesystem so it logs/displays what has files open there (if anything).

Still appears to be occuring, but it looks like the culprit is a buggy pulseaudio which respawns constantly when it is shutdown ... I will try disabling this, but it happens intermittently so it's difficult to find the cause.

Did you find a solution?
Could you tell me if you have ACL list enabled (CONFIG_TMPFS_POSIX_ACL)? Also, do you have any nfs mounts, or use pam_mount by any chance?

Same problem here on all my machines which have nfs mounts. Kerberised nfs, if that makes a difference. If I carefully kill all pids that have any handles to an nfs mount before reboot, seems to work ok.

As an example, typing reboot at a root prompt in KDE is guaranteed to cause a hang. Using KDE's own reboot option, which cleans up desktop processes before bouncing, works well, unless there's an underlying tmux session with additional handles.

Just to update this in case anybody else is interested, the problem appears to have stopped occurring after I updated pulse audio to v3. It appears that (as stated elsewhere on teh forums) there are some file handles left open. I also changed a config option in pulse audio settings (can't remember exactly what it was) which controls respawning.